Electronic devices often need to generate multiple power regimes while only being powered by a single source. For example, a laptop computer may only have a single battery but may need to produce power regimes with different supply voltages for the various components on the laptop. Furthermore, regardless of the need for multiple power regimes, electronic devices often need to condition the power that is delivered to them from an external source. Returning to the example of a laptop, the laptop processor contains sensitive electronics and exhibits a widely varying power demand based on how hard the processor is working. Simply plugging in a DC version of the mains voltage source is not an option because the processor will not be shielded from dips or surges in the power supply and the power supply will likewise not be able to keep pace with the rapid transitions in the power drawn by the processor. The aforementioned requirements are addressed by power converters.
Power converters receive power from a supply power regime and generate a regulated power regime. In one example, the power converter stabilizes a supply voltage in the regulated power regime and provides a varying current from the supply power regime in order to do so. Varying the current allows such a power converter to supply the varying power needs of any components or devices in the regulated power regime while keeping the supply voltage of the regulated power regime stable. Other power converters generate the regulated power regime by varying the voltage while keeping the current stable or vary both the current and voltage to keep an amount of power delivered to the regulated regime stable.
One class of power converters utilize the rapid switching of switches to transfer power in a controlled manner from a power source connected to their input to a load connected to their output. These power converters are often referred to as switching regulators or switched mode regulators. The frequency at which the switches are switched between a conductive and nonconductive state is referred to as the switching frequency of the converter and sets the amount of power transferred from the power source to the load. FIG. 1 provides one example of a switching regulator in the form of a buck converter 100. Buck topology is utilized when the input of the power converter is at a higher voltage than the output. As illustrated, voltage VIN is higher than the voltage VOUT. A load current iL is provided through an output filter comprising inductor 101 and capacitor 102 to load 103. Switches 104 and 105 are controlled by a driver circuit 106 and a feedback circuit 107 which receives information regarding the state of the load and/or power converter on node 108.
During regular operation, switches 104 and 105 alternately provide current from input VIN to the phase node 109 (also called the switch-node) and couple phase node 109 to ground. During the portion of the cycle when switch 105 is on, power to load 103 is being provided solely by energy stored in inductor 101 and capacitor 102. At the same time, energy is being stored in the parasitic inductance and capacitor of switch 105. When the cycle switches, the energy stored in these parasitics, the body diode of switch 105, and the power provided by switch 104 create undesirable ringing at phase node 109. The ringing is undesirable because it creates electromagnetic interference for the remainder of the electronic system of which the converter is a part, and because it increases the time it takes for the control circuit to determine the current state of the system and adjust the switching frequency in response to changes in the power requirements of load 103.
One option for reducing ringing on the phase node 109 is to add an R-C snubber circuit 110 including a capacitor and a resistor. The capacitor of snubber circuit 110 provides current so that the change in current through the inductor is not as rapid during a switching event. The resonant frequency of the R-C circuit is selected to critically damp signals operating at the frequency of the ringing. Snubber circuits thereby reduce ringing and are tuned to have minimal impact on signals of different frequencies. However, the snubber circuit reduces overall efficiency of the power converter by a couple of percentage points. As can be seen in FIG. 1, the capacitor of snubber circuit 110 must be charged and discharged during every switching cycle because the voltage across switch 104 fluctuates during each switching cycle. This decrease in efficiency is felt most prominently at light loads because the power consumed by the snubber circuit is proportional to the difference in the input voltage and the output voltage, and is not dependent on the load current iL.